The pinwheel-like drawing above is nothing but black and white lines. When you look at it the right way, though, something strange and beautiful happens: it begins to flicker. You may think it's just a regular old optical illusion at first, but actually, you're looking at your very own brain waves.
To see the optical illusion takes a little bit of work. Look at the pinwheel shape and then stare at a spot that's just a few inches away from it. When the pinwheel is in your peripheral vision, you should start to see the center flicker, kind of like a really bright star does. The effect also works as an afterimage. So once you find a spot that gets the flicker going, stare at it for about a minute and then look at a blank white wall. You'll see the inverse image of the pinwheel, flicker and all.
In a new paper, neuroscientists Rodika Sokoliuk and Rufin VanRullen argue that the flicker you're seeing is actually a physical manifestation of your brain's alpha waves. Alpha waves were the first neural oscillations detected by Hans Berger, the inventor of electroencephalography (EEG)—hence the name "alpha"—and represent rhythmic changes in brain activity. Typically, they cycle at a rate of about 10 Hz, or ten times a second. That frequency is partly how Sokoliuk and VanRullen made the connection between the flicker and the alpha waves. Asked to compare an artificial flicker with the one in the pinwheel, most subjects of their study selected one with a frequency of 9.1 Hz.
The frequencies matching could be a coincidence, but Sokoliuk and VanRullen also found a direct correlation between subject's alpha wave activity and the presence of the flicker. They explain in the paper's abstract:
The occipital alpha rhythm of the EEG was the only oscillation that showed a time course compatible with the reported illusion: when alpha amplitude was strong, the probability of reporting illusory flicker increased. The peak oscillatory frequency for these flicker-induced modulations was significantly correlated, on a subject-by-subject basis, with the individual α frequency measured during rest, in the absence of visual stimulation.